Question
Most of the leaves on my 15 year old tree are very small, about the sized of a thumb nail.  Around the base there are some suckers with leaves that are full sized and here and there on the tree there is a full sized leaf. They are not quite the normal green but are not yellow.

Thanks

Answer
The tree may have experienced some cold injury this last winter, depending on your geographic location.  The suckers and small leaves are indications that the tree went through some stress last growing season.  I would fertilize the tree with a slow release fertilizer and make sure it is watered during periods of dry weather.  If the suckers are coming from right at ground level, you may want to cut those off as they are also indications that the tree went through a stress and is trying to compensate with the sucker growth.